1. Field of the Invention
The present invention relates to an LCD projector, and more particularly, to an automatic gain control device of an LCD projector for converting an analog color signal received from a VGA card to a digital color signal.
2. Description of the Prior Art
LCD (liquid crystal display) projectors are frequently used for displaying RGB signals generated by a VGA card of a personal computer. Because RGB signals generated by different VGA cards have different voltage amplitudes, the RGB signals displayed by an LCD projector may appear differently when the RGB signals displayed by the LCD projector are generated by different VGA cards.
A prior art video AGC (automatic gain control) circuit used in many displaying devices can be used to adjust composite video signals which contain a reference signal related to the voltage amplitude of each video signal. But it can not be used to adjust RGB signals generated from VGA cards because these RGB signals do not have reference signals. Please refer to FIG. 1 and FIG. 2. FIG. 1 shows a composite video signal 10. FIG. 2 shows two analog RGB signals 20 generated by a VGA card. The composite video signal 10 comprises a brightness synchronization signal 12, a chromatic synchronization signal 14 (3.58 MHz), and an analog color signal 16. The amplitude of the brightness synchronization signal 12 is proportional to the amplitude of the analog color signal 16. The brightness synchronization signal 12 is used by a prior art video AGC device as a reference signal so that the analog color signal 16 can be completely converted into a digital color signal. The analog RGB signals 20 generated by a VGA card do not have such brightness synchronization signals, and thus cannot be adjusted by using prior art video AGC devices.